Through associative learning, neutral sensory stimuli are transformed into behaviorally relevant conditioned stimuli that signal behavioral contingencies. In operant conditioning paradigms, conditioned stimuli must be detected by sensory systems and relayed to appropriate association centers in the brain. In the somatosensory system, primary sensory relay cells in the trigeminal complex produce two major ascending and contralateral somatosensory pathways. One innervates the thalamus in the forebrain (trigeminothalamic) and the other innervates the deep layers of the superior colliculus in the midbrain tectum (trigeminotectal). The role of these two ascending pathways in the detection of somatosensory stimuli that are behaviorally relevant, such as conditioned stimuli in an operant conditioning task, is not known. Here we propose that the superior colliculus is a major sensory relay that serves to detect behaviorally relevant sensory signals in operant conditioning paradigms but with less analytical capabilities than the thalamic sensory relay. These two relay systems (trigeminothalamic and trigeminotectal) work together to detect and identify sensory conditioned stimuli. Therefore, in addition to its well-known role in orienting to novel stimuli, the trigeminotectal pathway would be an essential part of the sensory circuitry responsible for the detection of conditioned stimuli that signal behavioral contingencies requiring immediate operant responses. There are two major goals in this project. The first goal is to determine the response properties and neuromodulation of superior colliculus cells. The second goal is to characterize the activity and response properties of these cells as animals detect behaviorally relevant sensory stimuli during performance in an active avoidance task. The long term goal of this research project is to reveal the neural substrates of a complex form of emotional conditioning, active avoidance, that has direct relevance to many anxiety and mood disorders, such as phobias, social anxiety and depression.